1. Technical Field of the Invention
The present invention relates to integrated circuits, and more particularly integrated circuits comprising one or more acoustic resonators, preferably piezoelectric acoustic resonators, which can be used in signal processing applications, for example being used in a filtering function.
2. Description of Related Art
It is known in the prior art to produce acoustic or piezoelectric resonators arranged on interconnection levels or in interconnection levels. Conventionally, the acoustic resonators generally comprise an active element and a support which constitutes an acoustic mirror. Such resonators are thus secured to the integrated circuit while having to be acoustically or mechanically insulated from it. To this end, the supports for acoustic resonators make it possible to produce this insulation. In order to produce such insulation, a plurality of structures of a support for acoustic resonators have thus been envisaged.
In particular, one of these structures consists in producing a support comprising at least one bilayer assembly comprising a layer of high acoustic impedance material and a layer of low acoustic impedance material produced with the aid of a material with low electrical permittivity, as described in published PCT Patent Application WO 2004/051848, the disclosure of which is hereby incorporated by reference. In this way, the greater the difference in acoustic impedance is between the materials of the layers of the support, the more the acoustic waves liable to propagate in the integrated circuit will be reflected. The propagation of these waves is thus minimized in the integrated circuit.
Another possible structure consists in producing a support comprising an alternating sequence of at least two insulating layers with low acoustic impedance and at least two metal layers with high acoustic impedance stacked on one another on a substrate, as described in U.S. Pat. No. 6,542,054, the disclosure of which is hereby incorporated by reference. The metal layers are contained in auxiliary layers in order to avoid covering all the surface of the insulating layers, so as to obtain a support which has a planar surface. No indication is given about producing the connection of such an acoustic resonator within the integrated circuit.
Another of these structures consists in producing a certain number of acoustic resonators connected together via their electrodes as well as via a conductive layer in order to improve the filtering function of an integrated circuit, as described in United States Patent Application Publication No. 2004/0124952, the disclosure of which is hereby incorporated by reference. This conductive layer acts so as to electrically couple the entire architecture. No indication is given about producing the connection of such an acoustic resonator within the integrated circuit.
Another structure which is also possible consists in producing an acoustic resonator support which consists of stacking an air-filled cavity, which may be obtained by chemical or plasma etching, and a silicon nitride membrane which is arranged on such a cavity. The silicon nitride layer thus surrounds the entire air-filled cavity and lies directly in contact with the active element of the acoustic resonator. The air-filled cavity may be supported by a silicon oxide layer. In other words, this cavity is sandwiched between a silicon oxide layer and the silicon nitride membrane. There is also a passivation layer consisting of a dielectric material on top of the acoustic resonator, which is used to adjust the resonant frequencies of the acoustic resonators.
In particular, the resonant frequencies of the acoustic resonators are adjusted to the same frequency by a step of thinning this passivation layer, in order to obtain a satisfactory filtering function of the integrated circuit. This thinning step consists in abrading the surface of the passivation layer. Problems of bonding the membrane to the air-filled cavity may arise during this method, however, thus making the resonator non-functional. In general, such an architecture makes it difficult to carry out a method involving all sorts of liquids because of the membrane bonding problems. This architecture therefore makes it difficult to adjust the resonant frequencies of the acoustic resonators, and likewise does not make it possible to produce a solid membrane.
In order to produce such an architecture, it is a furthermore preferable to planarize the walls of the air-filled cavity in order to avoid the formation of a slot within the piezoelectric layer.
Such an architecture is described particularly in the scientific articles: “Integration of Bulk Acoustic Wave Filters: Concept and Trends,” by L. Elbrecht, R. Aigner et al., and “Integration of High-Q BAW Resonators and Filters Above IC,” by Marc-Alexandre Dubois et al., the disclosures of which are hereby incorporated by reference
In view of the preceding, there is a need in the art to produce an acoustic resonator which is connected to the rest of the integrated circuit and whose architecture has minimal roughness and makes a thinning step possible for adjusting the resonant frequencies of the acoustic resonator.